Various solutions have been used to clean and etch semiconductor wafers. Recently, general cleaning solutions, and particularly solutions designed for oxide etching, have been prepared containing ammonium fluoride as the primary cleaning or etching compound. In such instances, the semiconductor processing unit is designed to store the already prepared ammonium fluoride solution, which is then used when needed by the semiconductor processor.
Such solutions have been referred to as buffered HF solutions and consist of a mixture of hydrogen fluoride (HF), which is a weak acid, and a salt of this acid, ammonium fluoride (NH.sub.4 F). Initially this mixture was developed to buffer dilute HF so as to obtain a constant etch rate over an extended period of time (i.e., several days to weeks). Such solutions may be referred to as Buffered HF (BHF); the term BOE (Buffered Oxide Etch) is also used to refer to these solutions. Therefore, the appropriate solution (i.e., BHF or dHF) can be selected to optimize the specific etching and/or cleaning step to be performed.
It was discovered, however, that buffered hydrogen fluoride has slightly different properties than dilute HF, aside from the buffering, which can be used to a semiconductor processor's advantage in several occasions.
The major difference between BHF and dHF (dilute HF) is the different pH of the solutions. BHF has a pH of around 3 to 5, whereas dHF has a pH of around 1 to 2. Thus, the active species in the solutions is different.
The use of a BHF solution is attractive in some semi-conductor processing applications because: (1) resist is more "stable in" to BHF than to dHF (dilute HF) as a result of BHF's higher pH; (2) BHF has different etch selectivity than dHF (e.g., BHF is used when the BPSG (borophosphosilicate glass) versus thermal oxide has to be lower than the selectivity obtained in dHF; or BHF can also obtain the opposite if formulated right (i.e., it can also offer much higher etch rates for BPSG than thermal oxide); (3) BHF offers higher oxide etch rates than dHF for the same fluoride concentration; (4) BHF also offers higher nitride etch rates; (5) BHF terminates the surface differently than dHF; and (6) the higher pH of the BHF may in certain cases offer an improved particle performance depending on the particular action of the zeta potential.
In conventional BHF techniques, the solution is premixed and then employed in a wet processing unit. There are various problems associated with the use of premixed BHF. For example, particle formation tends to be an issue; such particles may compromise the integrity of the final product. Etch uniformity is limited due to the high viscosity and surface tension of BHF, the high etch rate, and, in the case of a wet bench, the continuous etching during insertion or removal of the wafers from the processing tanks and into the rinse tank. Also, because premixed BHF is typically used for an extended period of time, sometimes up to a week, there is the potential to introduce metallic impurities into the process liquids, which could again, compromise the integrity of the final product. Further, BHF tends to crystallize at temperatures below 16-17.degree. C. which poses a problem for commercial transportation of the premixed chemical from various locations (i.e., supplier to water processing plant). Because premixed BHF has a high viscosity and surface tension, a surfactant is generally required to improve its wetting properties and to reduce particle formation. Finally, with premixed BHF, one is limited to the premixed ratio of NH.sub.4 F to HF, typically mixing ratios of 6:1, 7:1 or 10:1, limiting flexibility in processing. The use of premixed BHF also is expensive.
Although there are significant benefits to using BHF to treat electronic component precursors, there is a need to reduce the problems associated with premixed BHF. The present invention addresses these as well as other needs.